Hydraulic fracturing is a widely used technique to stimulate the production of petroleum and natural gas from subterranean formations. After a wellbore is drilled into the reservoir rock, fracturing fluid, generally water containing suspended proppant, is injected in a manner to open a fracture into the exposed formation and place proppant within the newly formed fracture to prevent the complete closure of the fracture. This process increases the conductivity of the hydrocarbon-bearing subterranean formation and facilitates the production of previously trapped oil and gas from reservoirs and into the wellbore.
Liquefied gas fracturing (with fluids like CO2) can be used to alleviate many of the associated problems with water-based fracturing such as reduced permeability due water trapping, swelling and migration of water-sensitive clays, and reduced or eliminated surface fluid spills. However, there are limitations in commercial application. For CO2, equipment must be operated at elevated pressures above the triple point of carbon dioxide (i.e., greater than 75.1 psia) in order to maintain a liquid and pumpable state. Equipment is available for fracture treatments to mix proppant directly with liquid carbon dioxide based fracturing fluids. This equipment generally consists of a pressurized vessel and manifold system that blends the proppant into a liquid CO2 stream prior to the high-pressure pumps. Proppant is loaded into the CO2 blender where the unit is sealed and then filled with CO2. During the fracturing process, proppant is drawn into the fracturing fluid by either augers or gravity fed through a control valve.
Earlier efforts, as described in U.S. Pat. No. 4,374,545, provide for a batch process creating a proppant and LCO2 fracturing slurry. Each unit is capable of metering up to 20 tons of a single type of proppant and addresses the control of proppant supply through the use of a metering auger. LCO2 additions made to the tank allow for a flowable and vapor-free proppant slurry leaving the system as well as maintaining pressure in the vessel to prevent misdirected flow of CO2 from the main fracturing fluid stream back into the proppant supply.
The U.S. Published Patent Application U.S. 2015/0060065 A1 describes a control system, associated methodology, and apparatus for implementation of an eductor-mixer technique to provide the capability to inject and meter proppant material into a non-aqueous fracturing fluid stream. The system utilizes a solids-conveying liquid eductor instead of a conventional auger to mix and accelerate proppant within the main fracturing liquid stream. The control system utilizes at least one valve for controlling the flow of proppant from one or more pressurized proppant reservoir into the eductor; thereby mixing the material with the motive stream. Gas and/or liquid is fed to the top of the proppant reservoir to control the pressure inside the proppant reservoir. Modifying the pressure inside the proppant reservoir extends the range of achievable proppant flow rates from the reservoir into the eductor.
Since the use of reservoir pressure (with solids-handling valving) is used to manipulate the ultimate flow rate of material from the proppant reservoir, it is pertinent to determine the effect of pressure on the resulting proppant concentration. This would be difficult to achieve without the use of complex modeling and complete characterization of equipment at various treatment rates and proppant concentrations. If a densitometer is used to correct pressure values inside the proppant reservoir, hysteresis or time-delay in concentration readings can create the possibility of overshooting desired loadings. Equipment can be set up to incrementally adjust reservoir pressure to reduce the likelihood of overshooting concentration values, but at the cost of increasing the response time. To overcome the disadvantages of the related art, it is an object of the present invention to provide a method for adjusting static pressure inside the proppant reservoir for the purpose of providing timely changes in proppant concentration in the fracturing fluid, while reducing the probability of overshooting desired proppant concentration values. Specifically, the control mechanism developed utilizes a flow meter located on the main fracturing fluid stream (on either the clean or proppant containing side) and proppant loading sensor to guide static pressure changes in the proppant reservoir.
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.